SHA-1 vs SHA-256 Comparison

SHA-1 and SHA-256 represent two generations of the Secure Hash Algorithm family. SHA-1 (1995) has been deprecated due to demonstrated collision attacks, while SHA-256 (2001) remains fully secure. Migrating from SHA-1 to SHA-256 is one of the most important security upgrades for legacy systems.

Output and structure:

SHA-1 produces a 160-bit digest (40 hex characters) using 80 rounds with 32-bit operations on 512-bit blocks. SHA-256 produces a 256-bit digest (64 hex characters) using 64 rounds with 32-bit operations on 512-bit blocks. Despite SHA-256 having fewer rounds, each round is more complex and the wider output provides substantially more security.

Security status:

SHA-1's collision resistance has been practically broken. The SHAttered attack (2017) demonstrated a full collision, and chosen-prefix collisions were achieved in 2020 for under $50,000 in compute costs. This means an attacker can create two different documents with the same SHA-1 hash. SHA-256 has no known weaknesses. The best-known attack against SHA-256 is brute force, requiring approximately 2^128 operations for a collision, which is computationally infeasible.

Performance comparison:

SHA-1 is approximately 20-30% faster than SHA-256 in software. However, this performance difference is negligible in most applications where hashing is not the bottleneck. On CPUs with SHA-NI hardware extensions, SHA-256 is dramatically accelerated and can match SHA-1 speeds. The small performance gain of SHA-1 never justifies its security risk.

Migration strategy:

For TLS certificates, the industry completed its SHA-1 migration by 2017. For code signing, all major platforms now require SHA-256. For API authentication (HMAC), note that HMAC-SHA1 is still considered secure because HMAC's security does not depend on collision resistance, but migrating to HMAC-SHA256 eliminates questions. For Git, migration from SHA-1 to SHA-256 is underway with the SHA-256 object format.

Bottom line:

There is no legitimate reason to use SHA-1 in any new system. If you encounter SHA-1 in existing code, create a migration plan to SHA-256. The only acceptable exception is maintaining backward compatibility with systems you cannot control, and even then you should dual-hash (compute both SHA-1 and SHA-256) during the transition.